The present invention relates:
to a process for coating out of a metal alloy target at least one workpiece with an oxide of a metal alloy;
to a process for the production of a target comprising a metal alloy, with the alloy in the target being present substantially in a single phase;
to a preferred embodiment of the process for electrically conducting targets in general;
to a coating apparatus for cathodic arc evaporation with a gas inlet, connected to an oxygen supply, into the vacuum recipient and with at least one evaporation target comprising a metal alloy;
to a preferred embodiment of the apparatus with an electrically conducting target in general;
to a process for operating a cathodic arc discharge; and
to a use of said operating process.
Definition: In the following, the term xe2x80x9cphasexe2x80x9d is to be understood to mean the xe2x80x9ccrystallographic phasexe2x80x9d.
Oxides of metal alloys are conventionally deposited by reactive sputtering coating, electron beam evaporation, ion plating or by CVD processes as coatings. If attempts are made to deposit oxides of metal alloys with cathodic arc evaporation, numerous problems are encountered.
Attempts at controlling the movement of cathodic arc spots with the known means, such as with magnetic fields, which are successful with coating depositions of pure metal alloys or conducting nitrides are not successful for oxides of metal alloys. The reason for this failure is the known strong change of the secondary electron emission with the change of the target surface oxidation leading to a hysteresis of the state of the cathode surface.
In addition, the listed problems arising in alloy oxide coating are also characterized by the arc burning stationarily at particular target locations causing increased droplet emission which leads to stoichiometrically uncontrolled and even metallic droplet deposits.
Great interest with respect to fabrication technology exists in processes for coating workpieces with layers of insulating alloy oxides, in particular of stoichiometric ones, since these exhibit great hardnesses, which is known, for example, from EP-A-0 513 662, corresponding to U.S. Pat. No. 5,310,607 by the same applicant.
According to these references, hard material coatings (xe2x80x9cHartstoffxe2x80x9d in German) are suggested which are essentially formed by single or mixed crystal oxides of an alloy, specifically by (Al, Cr)2O3.
It is known from the field of reactive cathodic sputtering technology to control the poisoning of the metal target with non-conducting reaction product layers, in particular of interest here, with electrically insulating oxide layers, through reactive gas regulation. Such an approach was found to be counterproductive in cathodic arc evaporation. Lowering the partial pressure of oxygen and the consequent process control toward a metal mode during arc discharge evaporation increases the danger of localized burning and therewith the danger of droplet emissions and the jumping of the cathodic arc spot movement over very large distances on the target surface.
During reactive cathodic arc evaporation for the production of nitride coating, working in an atmosphere of excess nitrogen is recommended. Transferring this concept to oxide coating of the type of primary interest here, namely primarily to alloy oxide coating, but also generally to arc vapor depositions with insulating layers, such as, for example, with non-conducting metal oxide layers, does not lead to success since in the event of an oxide coating of the target or coating with non-conducting layers, the arc discharge frequently fails and, because of the poisoning insulation through the known ignition, mechanism, can no longer be reliably ignited.
These problems exist already in coating of workpieces with oxides of pure metals by means of cathodic arc evaporation but are significantly more pronounced if oxides of metal alloys are to be arc evaporated. The intensifications of the problems with alloy evaporation compared to metal evaporation per se are also known from nitride coating technology. In this connection reference is made to O. Knotek, F. Lxc3x6ffler, H.-J. Scholl; Surf. and Coat. Techn. 45 (1991) 53.
It is known from JP 5 106 022 to evaporate a Tixe2x80x94Al target by ion plating by means of a vacuum arc discharge and to deposit a TiAIN layer on a metal surface.
In xe2x80x9cCathodic arc evaporation in thin film technologyxe2x80x9d J. Vyskocil et al., J. Vac. Sci, Technol. A 10(4), July/August 1992, page 1740 a cathodic arc evaporation is described.
In xe2x80x9cEffects of target microstructure on aluminum alloy sputtered thin film propertiesxe2x80x9d, R. S. Bailey, J. Vac. Sci. Technol. A 10(4), July/August 1992, page 1701 sputtered layers are addressed.
With respect to cathodic arc evaporation in which the cathode itself is vapor-coated, reference is made to EP-A-0 285 745, corresponding to U.S. Pat. No. 4,919,968.
From EP-A-0513 662, corresponding to U.S. Pat. No. 5,310,607 oxide coating by means of crucible evaporation is known.
In principle, the use of cathodic arc evaporation for the production, in particular, of metal oxide layers and, in particular, of layers of alloy oxides is extremely desirable, for one reason because the cathodic arc evaporation leads economically to high coating rates. In principle, also an improvement of the process stabilization of reactive arc evaporation coating processes with insulating layers would be desirable.
It is the task of the present invention under all of its aspects, to permit coating workpieces, in particular with metal oxides and in particular also with oxides of metal alloys, but also generally with insulating layers from electrically conducting targets in a stoichiometrically controlled way, and to implement this by utilizing the advantages peculiar to cathodic arc evaporation, such as, for example, their high coating rate.
For coating by means of an oxide of a metal alloy, this is attained through a procedure according to the invention.
Surprisingly, it has been found that by using single-phase targets, in contrast to multi-phase targets, the cathodic arc spots move much more regularly on the target which avoids burn-in and drastically reduces the droplet density.
Although in some cases a limited quantity of other phases in the target is not disturbing, their fraction, according to another feature of the invention, should not exceed 30% or preferably 10%.
As will be explained in the following in conjunction with the examples, it was further found that generally the cathodic arc spot behavior during reactive arc evaporation of electrically conducting targets, in particular of metal targets, and deposition of an electrically insulating reaction product in the form of a layer can be divided into two characteristic domains. Generally, it is possible to differentiate clearly between a domain with relatively low partial pressure of the reactive gas and a few cathodic arc spots, which jump over relatively large areas of the cathode or target surface, and a second domain of relatively high partial pressure of the reactive gas in which many cathodic arc spots move significantly faster and/or over smaller areas on the cathode or target surface.
It has been found that the utilization of the second domain according to the invention, practically completely prevents the formation of droplets.
According to the invention a multi-spot domain is optimally utilized, i.e. the process operating point is selected to be directly at those partial pressures of the reactive gas at which the arc discharge would fail.
Stabilization of the process operating point can be carried out through observation and control, but preferably through regulation, with observation parameters preferably used or, in the case of a regulation, measured regulating variables as well as set variables during the control (open loop), or manipulated variables set by means of regulation technology in the case of a closed loop. The present invention is further based on the task of suggesting a process for the production of targets comprising a metal alloy with the alloy being present in the target essentially in a single phase.
According to another feature of the invention, the use of the above stated process on aluminum/chromium alloys has been found to be excellent.
According to a preferred embodiment a hard coating of said alloy with at least 5 at % (atomic percent) chromium, preferably with 10 to 50 at % chromium, is deposited, the latter being excellently suitable, according to the above cited EP-A-0 513 662, viewed from the aspect of its layer properties, for example for coating metal cutting tools.
The adhesion of said metal alloy oxide layer, in particular of the (Al, Cr)2O3 layer, as used on cemented carbide or ceramic bodies, such as are applied for use of metal cutting tools, is significantly increased and becomes more reproducible. Preferably a metallic intermediate layer of a metal/chromium alloy is deposited non-reactively but also with cathodic arc evaporation on the workpiece. Here too, preferably a target is used on which the metal/chromium alloy, at least primarily, is present in a single phase.
The layer succession is generated in the same coating chamber through the sequential contacting of the cathodic arc discharge onto the generally different targets and, for the deposition of the metal alloy oxide layer, the reactive gas oxygen is introduced into the treatment atmosphere.
As has been stated, the deposition of non-conducting metal alloy oxide layers within the scope of process stabilization is facilitated significantly since the above described xe2x80x9cmulti-spot domainxe2x80x9d is utilized.
But, according to another feature of the invention, this domain can be utilized generally for coating processes in which electrically conducting targets are cathodic arc discharge evaporated in a reactive gas atmosphere and out of a reactive product a coating is deposited, which product is electrically non-conducting or at least is a poorer conductor than the evaporated target material.
A coating apparatus for cathodic arc evaporation with a gas inlet, connected to an oxygen supply, into the vacuum chamber and with at least one evaporation target, in order to solve said task in terms of installation technology, is distinguished according to another feature of the invention.
Preferred embodiments of this apparatus or installation are specified according to the invention.
As mentioned, on said installation a second target can be provided, in particular with a metal/chromium alloy, preferably present primarily as a single phase, in order to deposit, apart from the metal alloy oxide layer, an adhesion-enhancing intermediate layer on the workpiece.
With respect to another feature of the invention, the following should be noted: because it was recognized according to the invention that generally during coating with layers that are electrically poorer conducting than the target material, a reactive arc evaporation process is advantageously stabilized in said multi-spot domain, the invention is directed toward an apparatus or installation in which generally an electrically conducting target is provided, preferably utilized control or regulating variables are specified with particular reference to the utilization of the discharge current frequency spectrum, as the measured regulating variable orxe2x80x94in the case of a controlxe2x80x94as the observed variable, a variable which is significant for the characteristics of occurring cathodic arc spots and their movement.